The overall goal of this proposal is to determine how specific genetic, biologic, and immunologic characteristics interact to predispose individuals to develop asthma. In that context, we propose to study a select population of infants with respiratory syncytial virus (RSV) bronchiolitis who are at substantial risk of developing asthma. RSV infection in the first year of life is a substantial risk factor for the development of persistent wheezing. The interaction of RSV with the host epithelial-immune system and underlying genetic background is unclear. Accordingly, Aim I proposes to evaluate the association between an atopic genotype, the R576 IL-4 alpha receptor mutation, and the development of an asthmatic phenotype in a prospective cohort of infants with RSV bronchiolitis. Given the appropriate risk factors, RSV illicits an immune response characterized by inflammatory cell influx, especially T cells, into the airway. Accordingly Aim II proposes to evaluate the effect of airway inflammation, demonstrated by persistent expression of the beta chemokine, RANTES, in airway epithelial cells, in a prospective cohort of infants with RSV bronchiolitis who are at risk of developing an asthmatic phenotype. This possibility is supported by evidence of increased RANTES expression (by mRNA stabilization) in human tracheal epithelial cells infected with RSV in vitro and in upper airway epithelial cells from infants with RSV bronchiolitis. The T cell immune response following viral infection appears to be primarily Thl-type, however in the setting of RSV bronchiolitis this is less clear. Accordingly, Aim III proposes to assess the T cell profile,, Th1 vs. Th2 phenotype, as defined by cytokine expression and other phenotypic markers, in a prospective cohort of infants with RSV bronchiolitis who are at risk of developing an asthmatic phenotype. Therefore, this proposal will lead to a better understanding of the interaction of genetic, biologic, and immunologic factors following RSV infection which lead to the development of asthma in early life.